Feeding rate and meal patterns in the laboratory rat

Physiology &Behavior, Vol. 32, pp. 369-374. Copyright©PergamonPress Ltd., 1984. Printedin the U.S.A.

0031-9384/84$3.00 + .00

Feeding Rate and Meal Patterns in the Laboratory Rat P. G. C L I F T O N

Ethology and Neurophysiology Group

D. A. P O P P L E W E L L A N D M. J. B U R T O N

Laboratory of Experimental Psychology, University o f Sussex, Brighton, U.K. R e c e i v e d 22 J u n e 1983 CLIFTON, P. G., D. A. POPPLEWELL AND M. J. BURTON. Feeding rate and meal patterns in the laboratory rat. PHYSIOL BEHAV 32(3) 369-374, 1984.--Many manipulations used in the study of feeding (e.g., changes of food taste or texture, anorectic drugs) affect the rate of food consumption. Consequent changes in meal patterning might reflect either direct effects of the manipulation or alternatively might result from indirect effects of the changed rate of intake. In the experiment reported here a direct reduction in the permitted rate of food intake resulted in a clear reduction of meal size and an increase in meal frequency in rats. We explore the extent to which this finding is predicted by quantitative models of the regulation of food intake. Rat Meal size Feeding rate Obesity Fenfluramine

Schedule-induced drinking

A number of techniques commonly used in the study of feeding may lead to changes in the rate at which an animal feeds; obvious examples are of lesions which slow intake rate (e.g., [21]), drugs such as fenfluramine which have similar effects on feeding rate (e.g., [3,6]) and changes in the palatability of food or the amount of work required to obtain each item of food (e.g., [8,20]). In many of these experiments changes in meal patterning have been observed. These changes may arise directly from the effects of the experimental manipulation or indirectly from the changes in intake rate. The possibility that meal parameters are not independent of the rate of food intake is of importance, since in many of the examples quoted intake rate reduction below control levels is typically associated with a pattern of smaller and more frequent meals. Attempts to modify intake rate are also of interest because feeding, at least as studied in the laboratory, often involves short periods of high intake rates. Although some soluble carbohydrates may be absorbed within 2-5 minutes of the beginning of a meal [18], the greater fraction of calories ingested will be absorbed after feeding has ceased. In such a situation an animal is likely to have some difficulty in judging an appropriate stopping point for the current meal [9]. The importance of delays in absorption should be greatly reduced if intake rate were slowed. Finally the effect of feeding rate on consumption patterns is of interest because of the limited evidence that feeding rate in obese people may be abnormal [19] and because restraint of eating rate is often recommended as one part of dietary management in the obese [ 14]. In the experiment to be described here rats were fed

369

Conditioned satiety

Stimulation

standard 45 mg Noyes pellets. Although the rat did not have to perform an operant response (e.g., bar press) for these pellets, only one was available at a time and a small delay (0 to 20 sec) could be interposed between the removal of one pellet and the delivery of the next. This rather simple procedure has not been used before as far as we are aware. It clearly carries the risk that stereotyped, schedule induced behaviour might appear; drinking is an obvious possibility and we therefore monitored drinking patterns throughout. In addition to the experimental results we have used a quantitative model of intake control in the rat [5] to explore the likely magnitude of effects of feeding rate on meal parameters given certain basic assumptions about, for example, the magnitude of delays between the ingestion and absorption of nutrients. METHOD Seven male hooded rats (age 12 weeks, body weight range 240-279 g) were housed singly in cages (45×30×30 cm) in which both food (45 mg Campden pellets) and tap water were freely available. There was also a small ( 1 5 x i 0 × 8 cm) open-top nest box in the corner of each cage, diagonally opposite to the food hopper. The cages were held in a single experimental room, maintained at 22-24°C, which provided visual but not auditory isolation of the rats from one another. The room was also maintained on a 12 hr:12 hr L/D cycle, with lights offat 19.00 hr; the rats were handled and weighed daily at 17:00 hr. The animals were allowed l0 days to habituate to these conditions before the experiment began. Feeding and drinking were recorded using a micro-

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FIG. 1. (a) A log-survivorship plot for intervals between pellets. The plot was produced by combining the data for each animal on the day with no delay in pellet delivery. Note the very marked change in slope at about 100 seconds. (b) A long-survivor plot of meal size for the same data--note the slight convexity which indicates a tendency towards meals of a typical size.

computer based system [6]. Briefly, the system timed (to 0.1 sec resolution) the removal of a single 45 mg pellet from a food hopper or the activation of a drinking sensor, such that a continuous record of feeding and drinking was available. The food pellet dispenser, which normally replaced pellets immediately, was fitted with an additional timer delay circuit which could hold back the delivery of the next pellet for between 0 sec and 20 sec; however because the delivery mechanism took between 1 and 2 seconds to operate the actual delays varied between 1 and 22 sec. In the following text we use the delay values to identify the different experimental conditions. Four delay values were used in the experiments (0 sec, 8 sec, 14 sec, 20see) and were chosen to produce feeding rates comparable to those found following fenfluramine administration [6]. Each delay condition began at 18.00 hr and lasted until 07.00 hr the following day (i.e., 13 hours). At all other times the delay was 0 sec. Each animal received all delays, 72 hours separated delay conditions and the orders of conditions were counterbalanced (c.f., [6]). Feeding and drinking patterns were extracted as described by Burton et al. [6], and analysis of variance was used to assess differences in meal patterning across the different delay conditions. RESULTS

Experimental Findings The 13 hour periods during which pellet delivery was delayed were extracted from the remainder of the data and analysed using standard microstructure techniques (e.g., [6]). Log-survivor plots (Fig. I) of the intervals between successive pellets suggested that a suitable meal criterion would be 2 minutes. This value was a little longer than the most

obvious break-point on the plot and was deliverately chosen to reduce the number of missassigned meals [17]. Although we examined log-survivor plots for each rat on each schedule we decided that the choice of slightly different criteria to distinguish between inter and intra-meal intervals for each individual would cloud subsequent interpretation of the data. Figure 2 shows a clearly significant decrease in meal size with increasing delay, F(3,18)=3.63, p<0.03, and Fig. 3 shows an equally clear increase in meal frequency, F(3,18)=7.43, p<0.001. There was no change in total intake related to delay time, as might be expected from the opposite directions of the effects on meal size and frequency. Although correlations of meal size with both the preceeding and following interval were calculated, these proved to be both low and variable as would be expected from the very limited number of meals occurring during a period as short as 13 hours. The conclusions related to meal size and frequency were not dependent on the particular interval criterion used during the analysis. Criteria of 5 and 10 minutes produced very similar trends in meal size and inter-meal interval, although at 20 minutes the relationship of meal size to feeding rate was weaker and non-significant. As is clear from the log-survivor plot a 20 minute criterion falls on a linear part of the plot, indicating no significant shift in the likelihood of taking a pellet 20 minutes after the last one was consumed. Total drinking in the 13 hour session also showed no relationship to the imposed feeding rate. The apparent rise for the 20 sec delay group was non-significant (Table I) and resulted from the one animal in the study which showed some evidence of schedule induced drinking; it drank more than 1000 times during this session. This overall lack of schedule induced drinking is illustrated by the proportion of transitions from feeding to drinking as a proportion of total drinking transitions which, although it rose at slower feeding

F E E D I N G RATE AND MEAL SIZE

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delay(s) FIG. 2. The effect of delay time on meal size. Meal size is shown as the number of pellets consumed. Each pellet had a calorific value of approximately 212 calories. A standard meal criterion of 2 minutes was used in each case. Error bars are SE of the mean, the effect was significant, F(3,18)=3.63, p<0.03.

rates, never exceeded 25 percent of the total (Table 1). However the proportion of drinking that occurred within meals did increase markedly, and together with the data on transitions, demonstrates a switch from a pattern in which meals terminated before drinking began, to one in which bouts of feeding and drinking were interspersed within a meal.

Simulations The simulations presented here were derived from the model of the rat feeding developed by Booth (summarised in [5]). The simulations were programmed in the language " C " and copies are available from the first author. Whether feeding occurs or not is controlled by net body energy flow; this is estimated by taking the difference between absorption from the gut and metabolic consumption. Feeding is initiated by the model at one level of energy flow (onset value) and stops at another higher level (offset value). This ensures that the model feeds in meals rather than jittering, or eating only in - n i b b l e s , " as it would if only a single threshold was present. In these simulations the feeding onset value was - 2 0 calories/minute and the offset value was initially +20 calories/minute. To this basic meal taking mechanism are added

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delay(s) FIG. 3. The effect of delay time on inter-meal interval. The effect was again significant, F(3,18)=7.43, p<0.001.

diurnal changes in metabolic rate and gut clearance functions which proved to be of little importance here. Two other additions proved to be crucial. The first of these was the estimated delay between the ingestion and the absorption of food. The value chosen here, and close to those typically used by Booth, was 3 minutes. The second crucial addition was the algorithm used by Booth [5] to simulate the effects of conditioned satiety. It works by measuring the peak energy flow after a meal and comparing this value to some previously specified target value, which in this case was the initially specified feeding offset value (+20 calories/minute). Initially the peak flow rate is likely to be higher than the feeding offset value because of delays in absorption. The size of the next meal is altered so as to more nearly achieve this target by altering the offset value in the appropriate direction. This procedure allows the model to respond to differences in the caloric density of the diet with appropriate changes in meal size in much the same way as can a real rat. However, as will be seen in the results presented below, changes in feeding rate that modify meal size are also compensated for by this mechanism. Figures 4 and 5 present the size of the first four simulated meals following changed feeding rates. In the simulations in which conditioned satiety was omitted a reduction in feeding rate also reduced meal size, with the extent of the reduction dependent on the size of the rate reduction. This remained true until very low rates were simulated (200 cal/min); here

372

( ' I . I F T ( ) N I;1 ,41. TABLE 1 FEEDING, DRINKING AND THEIR I N T E R R E L A T I O N S H I P S 0

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473 (60)

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216 132)

297 (39)

333 (27)

456 (45)

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Percent feed to drink transitions

4.4 (1.4)

17.9 (6.7)

17.4 (3.4)

24.1 (5.2)

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Proportion of drinking within meals

0.10 (0.06)

0.50 (0.10)

0.61 (0.07)

0.74 (0.03)

18.52 <0.001

Feeding records are of the mean number of pellets taken and drinking the mean number of sensor activations. Drinking within meals was defined as drinking occurring between food pellets that were separated by less than the 2 minute interval criterion. The parenthesised figures below each mean give the standard error. All F ratios have 3,18 degrees of freedom.

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FIG. 4. Sizes of the first 4 meals following a reduction in feeding from 1200 to 200,400,800 or 1200 cal/min. The effect were simulated using Booth 1978 model which is described in the text. Meal size is given in kcals (1 kcal is approximately 5 45 mg food pellets).

FIG. 5. A second set of simulations of meal size following a change in the rate of intake in which, in addition, the effects of conditioned satiety were included. Axes are as in Fig. 4.

meal size began to increase again because o f the sizeable demands of metabolic rate during these greatly e x t e n d e d meals. When the same simulations were p e r f o r m e d with the addition o f the conditioned satiety algorithm an entirely different result was obtained. A reduction in meal size was only present for the first meal after the rate change, and was then entirely c o m p e n s a t e d for by the model. Thus, as might have been anticipated, the model responds to a change in the rate of feeding precisely as it would to a change in caloric density; in fact to the model they appear as the same thing. Additional simulations which are not presented here demonstrated that a d e c r e a s e in the delay in gut emptying reduced the slope of the relationship b e t w e e n meal size and rate of feeding when conditioned satiety was not included. The addition o f diurnal changes in metabolic rate and gut emptying constant [5] also did not affect the conclusion that

the model without conditioned satiety predicted the observed results, while that with the algorithm for conditioned satiety included was unable to do so. DISCUSSION

The experiment presented here shows clearly that intake rate can be an important determinant of meal patterning. In particular it shows that a decrease in permitted feeding rate will cause a reduction in meal size and an increase in meal frequency w h e n food is available ad lib. This finding has an important implication for studies which e x a m i n e meal patterning following a manipulation (such as drug administration) which may be directly affecting intake rate. It would be rash to assume that the changes in meal patterning give a direct indication of the motivational processes underlying the action of the drug. Thus fenfluramine causes a reduction

F E E D I N G RATE A N D M E A L SIZE

373

in feeding rate together with a reduction in meal size [2, 3, 6] which may be interpreted as indicating that fenfluramine exerts a specific effect on processes that lead to meal termination [3]. The present data suggests that a more parsimonious interpretation might be that this change is secondary to the effect on feeding rate which itself might result from the more general behavioural changes induced by fenfluramine. A number of quite different explanations of our observed reduction in meal size as intake rate is slowed may be envisaged. Firstly it might be that, because of a simultaneous production of schedule induced drinking [16], feeding was suppressed by a substantial increase in the volume of total stomach contents. This explanation is unlikely for although changes in the distribution of water taken during and between meals were observed, they had no effect on total fluid intake. The excess drinking within meals did not have the pronounced characteristics of schedule induced drinking, in particular it occurred only during a low proportion of interpellet intervals. A substantial reduction in schedule induced effects as the level of food deprivation is reduced has been observed by several workers (e.g., [16]). The effects of the changed distribution of drinking might potentially either increase or decrease meal size. Changes in stomach volume or the osmotic pressure of the ingested food would probably tend to reduce meal size, whereas drinking during a meal consisting of dry pellets might tend to augment meal size. Such effects could be eliminated by repeating our experiment using a liquid diet that did not require separate provision of water. Two further possibilities which explain the result in terms of interactions within the feeding system alone deserve consideration. The first of these was examined in simulations using Booth's [5] model. It is, briefly, that a slowed intake rate allows satiety to develop more effectively during the course of a meal, and thus reduces meal size. This model is intuitively attractive and, at the quantitative level, produced realistic simulations of the effects that we observed. However it does face several problems. In particular changes in permitted feeding rate seem closely equivalent to changes in caloric density of the diet which do not affect feeding rate. The addition of Booth's conditioned satiety algorithm to the model, permitting it to adjust meal size to changes in the caloric density of the diet, also allowed it to adjust to changes in feeding rate: indeed in the model they are equivalent. There is no obvious reason why an animal should not be able to learn about changes in the ability of it's usual food to produce satiety during a meal, when it clearly can learn about the satiating consequences of a novel food [4], especially since it is only in the laboratory that a particular food is likely to remain calorically identical with time. Many authors (e.g., [9,13]) have suggested that feeding, once initiated, may be sustained by a positive feedback process and this idea suggests an alternative explanation of our experimental results. In Booth's model an effect of essentially this kind is included by having one threshold value of net energy flow at which feeding starts and a second higher value at which it stops. In current versions of the

model this switch in threshold occurs instantaneously as feeding begins. There is, however, some evidence that this may be an oversimplification and that the effect actually builds up over the first part of a m e a l [15,21]. It seems most likely that a process of this kind is initiated by each food item taken, first increasing and then decreasing again. The idea of interacting incremental and decremental processes following the consumption of each food item has not received much attention, however the behavioural literature contains at least two other examples of models based on processes initiated by each of a series of acts. Hinde used such a model when describing the organisation of mobbing calls and song in the chaffinch ([11] and [10] respectively) as did Beach and Whalen [ 1] in their description of copulation in the male rat. Here the way in which the effects of a number of food items summated would depend crucially on their separation from each other, with increased separation allowing less effective summation across acts, thus generating the prediction of lowered meal size with a lowered feeding rate. However this hypothesis remains less attractive in that there is, as yet, no independent assessment of the likely magnitude and duration of this positive feedback process in rats, and it therefore cannot be modelled quantitatively. The experimental effect observed here is also of interest in relation to the work of Collier and others on the influence of variation in the work required to obtain food on meal patterns (e.g., [8]). They distinguish between procurement and consumption costs which are respectively the costs of initiating a meal and the costs of consuming items that make up a meal. Their data (e.g., [7]) show that changes in procurement cost lead to large changes in the patterning of food intake in such a direction as to minimise the imposed cost increase. Changes in the cost of consumption have much smaller and less reliable effects. This is not unexpected since changes in meal patterning are unable to alter the overall cost of intake requirements in this case. However in a recent report by Kaufman and Collier [12] rats were presented with either hulled or fion-hulled sunflower seeds, effectively presenting them with the same diet at two different intake rates. Again meal size was smaller on the effectively lower intake rate, although this conclusion may have been influenced by the fact that a third source of food was available in both conditions. In summary a reduction in intake rate is associated with a reduction in meal size. This experimental finding could be accounted for within the feeding system in two rather different ways. Firstly it is possible that the slower intake rate allows the animal to "catch u p " with the satiating consequences of feeding. Secondly it may be that the greater spacing of items within the meal disrupts the normal positive feedback processes that sustain the meal.

ACKNOWLEDGEMENTS

We are grateful to the Medical Research Council (U.K.) for financial support and to Peter Slater for comments on a preliminary version of the manuscript.

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't'I.IFI't)N I:1 .41.

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